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  #1  
Old 06-24-2009, 07:21 AM
singapore1 singapore1 is offline
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Default 2 diodes in parallel

In rectifier circuit, if the forward current is too high for a single diode to handle, what should one do? i will think of using 2 same diodes in parallel to solve this problem. However i get response that i should not do this. Can anyone explain why? I do know that there's no 2 EXACT same diodes practically and the current will not divide equally. However I don't see any issue here, even if the current divides 70% vs 30% because of mismatch of diodes, it will not cause any harm to the diodes as long as 70% of total current does not exceed the power handling of the diode.

If i use 2 same diodes in parallel, i do not expect diode mismatch will cause the entire current to flow through 1 of the diodes or result in high current mismatch. I expect maybe worst case a 60% vs 40% current mismatch.
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Old 06-24-2009, 01:39 PM
steveb steveb is offline
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Quote:
Originally Posted by singapore1 View Post
In rectifier circuit, if the forward current is too high for a single diode to handle, what should one do? i will think of using 2 same diodes in parallel to solve this problem. However i get response that i should not do this. Can anyone explain why? I do know that there's no 2 EXACT same diodes practically and the current will not divide equally. However I don't see any issue here, even if the current divides 70% vs 30% because of mismatch of diodes, it will not cause any harm to the diodes as long as 70% of total current does not exceed the power handling of the diode.

If i use 2 same diodes in parallel, i do not expect diode mismatch will cause the entire current to flow through 1 of the diodes or result in high current mismatch. I expect maybe worst case a 60% vs 40% current mismatch.
I think you can do it, but the benefit is not all that much. Since you need to allow for mismatch, you don't even double your current capability.

A better solution is to just get a diode that can handle the current: -they are certainly cheap enough. I good trick is to use a BJT power transistor. If you connect the collector and the base together, you have your diode.
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Old 06-24-2009, 02:24 PM
russ_hensel russ_hensel is offline
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If you do have to do it put a resistor in series with each diode, drop .5 to 1.5 volts there at full current and you should ballance the current fairly well, esp if the diodes are fairly closely matched. Better -- bigger diode as indicated above.
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Old 06-24-2009, 02:43 PM
gerty gerty is offline
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One thing we use when we need a high current diode is a high current bridge rectifier. All you need to do is use 2 pins, which 2 pins depends on which polarity you need. 25 amp bridges are usually easier to find than an actual 25 amp diode.
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Old 06-24-2009, 09:36 PM
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THE_RB THE_RB is offline
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With normal silicon rectifiers the forward voltage knee is pretty soft so you can parallel them ok without needing resistors. The Vf curve can be as wide as 0.5v at 1mA to 1.2v at full rated current. It's still good to choose the diodes by measuring the Vf with your multimeter and try to match them.

You shouldn't parallel schottky power diodes as their Vf drops considerably when they get hot, which can lead to the hotter diode starting to take all the current and destroying itself.
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